Thin films of carbon nanotubes (CNTs) are an attractive option for the conductive layer in a next generation of thin-film transistors (TFTs) for low-power, high-performance, flexible electronics. Various means of fabricating CNT thin films are possible – from self-assembly to inkjet printing – but using aerosols to make such high performance devices has been largely unexplored… until now.
Jacques Lefebvre and Jianfu Ding of Canada’s National Research Council believe aerosols could hold unique advantages for fabricating CNT thin films [Lefebvre and Ding, Materials Today Communications 10 (2017) 72].
“There are a limited number of printing methods suitable for CNT thin films,” says Lefebvre. “The method we have developed tries to address these issues by exploring the potential of aerosol deposition in an over-looked regime.”
The simple approach starts with a solution of polymer-wrapped semiconducting CNTs (or sc-SWCNTs). This solution is fed into a commercial sub-micron droplet generator, which can produce droplets with diameters of just 1-2 microns. By tuning the concentration of the sc-SWCNT solution, droplets containing a single nanotube can be created. By applying an electrostatic field to the ejection nozzle and a substrate, which could be a conductor or an insulator, droplets containing SWCNTs can be deposited onto the surface.
“The process has three main attributes,” explains Lefebvre. “Micron-sized droplets accommodate only a few CNTs per droplet; an electrostatic field draws charged droplets onto a target substrate; and very mild flow minimizes the forces/interactions on droplets.”
The result is thin films of sparsely assembled, 1 micron-long nanotubes with densities of 10-50 per square micron. The researchers demonstrate the usefulness of the thin films as the channel material in TFTs, which show excellent switching performance – meaning they can carry large currents when ‘on’ and use little current when ‘off’.
The performance of aerosol-deposited TFTs compares well with other printing methods, but offers some distinct advantages.
“To our knowledge, this method is the only one that can deposit CNTs onto surfaces such as Teflon and other fluorinated or low surface energy dielectrics,” points out Lefebvre.
When CNT transistors are fabricated on hydrophilic surfaces, for example in oxide dielectrics, air exposure alters electrical performance. Encapsulation is the only viable option, but using the new aerosol process allows the fabrication of transistors without the need for encapsulation.
The process is also easy to scale up and offers the possibility of covering large areas very quickly. Ultimately, the approach could produce devices in large quantities at low cost.
“We are currently looking at the devices that would benefit most, initially in niche applications that do not have significant production constraints,” says Lefebvre.